Lyman Break Galaxies

Observations of distant objects, notably high-redshift star-forming ( Lyman-break ) galaxies and absorption line systems on the line of sight to quasars, give some information on chemical evolution at epochs not too far from when the first stars and most galaxies were presumably formed. Other information comes from two related effects ... 

Fig. 12.9. Composite spectrum of Lyman-break galaxies showing a combination of interstellar and stellar absorption lines, P Cygni features and nebular emission lines, dominated by Lyman-a. After Shapley et al. (2003).

Ejection via early winds is inferred in the enriched intracluster medium. Observations of Mpc-scale holes around Lyman break galaxies, detected via studying absorption of the IGM towards background quasars, support an explanation in terms of early winds from L galaxies. The so-called cooling catastrophe in galaxy formation theory, which results in overly luminous massive galaxies can be avoided if early winds eject an amount of baryons comparable to that retained in stars. 

A level of metal enrichment of 10-3 to 10 2 of solar in regions of the IGM with N(H I) > 1014 cm-2 may still be understood in terms of supernova driven winds from galaxies. The work of Aguirre et al. (2001) shows that such outflows which, as we shall shortly see ( 4.5) are observed directly in Lyman break galaxies at z = 3, may propagate out to radii of several hundred kpc before they stall. However, if O VI is also present in Lya forest clouds of lower column density, as claimed by Schaye et al. (2000), an origin in pregalactic stars at much earlier epochs is probably required (Madau, Ferrara, Rees 2001). 

Figure 21. All the 3300 galaxies from Figure 20 are included in this colour-colour plot. The shaded region shows how the 140 candidate Lyman break galaxies are selected for subsequent spectroscopic follow-up. The symbol size is proportional to the object magnitude circles denote objects detected in all three bands, while triangles are lower limits in (Un — G) for U dropouts.

C IV, N V). The lines are fully resolved so that column densities can be derived from the analysis of their profiles. From these data Pettini et al. (2002b) were able to piece together for the first time a comprehensive picture of the chemical composition of the interstellar gas in a Lyman break galaxy (Figure 25) and examine the clues it provides on its evolutionary status and past history of star formation. 

FIGURE 26. Example of a NIRSPEC K-band spectrum of a Lyman break galaxy from the survey by Pettini et al. (2001). The objects targeted typically have K = 21 (on the Vega scale) and remain undetected in the continuum. However, the nebular emission lines of [O III] AA4859,5007, [O II] 3727 (not shown), and H/J usually show up clearly with exposure times of 2-3 hours. The dotted line is the la error spectrum. 

Evidently, Lyman break galaxies span a wide range of ages. One fifth of the sample considered by Shapley et al. (2001) consists of objects which apparently have just collapsed and are forming stars on a dynamical timescale ( 35Myr). As we have seen,... 

Predicted All Baryons (BBNS) Metals synthesised in Lyman Break Galaxies 0.088 0.035... 

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